The Fractional Excretion of Urea (FEUrea) is a critical diagnostic tool used by clinicians to differentiate between prerenal azotemia and intrinsic acute kidney injury (AKI). While the Fractional Excretion of Sodium (FENa) is often the first test considered, FEUrea provides superior diagnostic accuracy in patients who have recently received diuretics.
FEUrea Calculator
What is the FEUrea Calculation?
The FEUrea calculation measures the percentage of urea filtered by the kidney that is eventually excreted in the urine. It compares the clearance of urea to the clearance of creatinine. Because urea handling in the proximal tubule is less affected by diuretics (like Lasix) than sodium handling, FEUrea remains a reliable marker even when FENa is skewed.
The FEUrea Formula
The mathematical representation of the calculation is:
FEUrea = [(Urine Urea × Serum Creatinine) / (Serum Urea × Urine Creatinine)] × 100
Interpreting Your Results
Once you have performed the feurea calculation, the result is expressed as a percentage. Generally, clinicians look for the following thresholds:
- FEUrea < 35%: Strongly suggests a Prerenal cause. This indicates the kidneys are functioning well but are under-perfused (e.g., dehydration, heart failure, or blood loss). The body is aggressively reabsorbing urea to maintain volume.
- FEUrea > 35%: Suggests Intrinsic AKI (such as Acute Tubular Necrosis - ATN). This indicates that the renal tubules are damaged and cannot effectively reabsorb urea, leading to a higher percentage being "wasted" in the urine.
Why Use FEUrea Over FENa?
In many clinical settings, patients with acute kidney injury are already on loop diuretics. Diuretics work by inhibiting sodium reabsorption in the Thick Ascending Limb of the Loop of Henle. This artificially increases the amount of sodium in the urine, leading to a high FENa (>1%) even if the patient is actually in a prerenal state.
Because urea is primarily reabsorbed in the proximal tubule—a site largely unaffected by loop diuretics—the FEUrea calculation provides a "cleaner" look at renal function in these complex patients.
Clinical Limitations
While the feurea calculation is a powerful diagnostic aid, it should not be used in isolation. Factors that can influence urea levels include:
- High protein intake or gastrointestinal bleeding (increases BUN).
- Severe malnutrition or liver disease (decreases BUN).
- The use of osmotic diuretics (like mannitol) or SGLT2 inhibitors.
Always correlate the FEUrea result with the clinical history, physical exam (volume status), and other laboratory findings like the BUN/Creatinine ratio and urine microscopy.